In vivo gene therapy is a strategy in which nucleic acid, usually in the form of DNA, is administered to modify the genetic repertoire of target cells for therapeutic purposes. This can be accomplished efficiently using a recombinant adenoviral vector encoding a so-called "therapeutic gene". A therapeutic gene is generally considered a gene that corrects or compensates for an underlying protein deficit or, alternately, a gene that is capable of down-regulating a particular gene, or counteracting the negative effects of its encoded product, in a given disease state or syndrome. Recombinant adenoviral vectors have been used to transfer one or more recombinant genes to diseased cells or tissues in need of treatment. As reviewed by Crystal, Science, 270, 404-410 (1995), such vectors are preferred over other vectors commonly employed for gene therapy (e.g., retroviral vectors) since adenoviral vectors can be produced in high titers (i.e., up to 10.sup.13 viral particles/ml), and they efficiently transfer genes to nonreplicating, as well as replicating, cells. Moreover, adenoviral vectors are additionally preferred based on their normal tropism for the respiratory epithelium in cases where the targeted tissue for somatic gene therapy is the lung, as well as for other reasons (see, e.g., Straus, In Adenoviruses, Plenan Press, New York, N.Y., 451-496 (1984)); Horwitz et al., In Virology, 2nd Ed., Fields et al., eds., Raven Press, New York, N.Y., 1679-1721 (1990); Berkner, BioTechniques, 6, 616 (1988); Chanock et al., JAMA, 195, 151 (1966); Haj-Ahmad et al., J. Virol., 57, 267 (1986); and Ballay et al., EMBO, 4, 3861 (1985)).
There are 49 human adenoviral serotypes, categorized into 6 subgenera (A through F) based on nucleic acid comparisons, fiber protein characteristics, and biological properties (Crawford-Miksza et al., J. Virol., 70, 1836-1844 (1996)). The group C viruses (e.g., serotypes 2 and 5, or Ad2 and Ad5) are well characterized. It is these serotypes that currently are employed for gene transfer studies, including human gene therapy trials (see, e.g., Rosenfeld et al., Science, 252, 431-434 (1991); Rosenfeld et al., Cell, 68, 143-155 (1992); Zabner, Cell, 75, 207-216 (1993); Crystal et al., Nat. Gen., 8, 42-51 (1994); Yei et al., Gene Therapy, 1, 192-200 (1994); Chen et al., Proc. Natl. Acad. Sci., 91, 3054-3057 (1994); Yang et al., Nat. Gen., 7,362-369 (1994); Zabner et al., Nat. Gen., 6, 75-83 (1994)). Other groups and serotypes include, but are not limited to: group A (e.g., serotypes 12 and 31), group B (e.g., serotypes 3 and 7), group D (e.g., serotypes 8 and 30), group E (e.g., serotype 4) and group F (e.g., serotypes 40 and 41) (Horwitz et al., supra).
In terms of general structure, all adenoviruses examined to date are nonenveloped, regular icosahedrons of about 65 to 80 nanometers in diameter. Adenoviruses are comprised of linear, double-stranded DNA that is complexed with core proteins and surrounded by the adenoviral capsid. The capsid is comprised of 252 capsomeres, of which 240 are hexons and 12 are pentons. The hexon capsomere provides structure and form to the capsid (Pettersson, in The Adenoviruses, pp. 205-270, Ginsberg, ed., (Plenum Press, New York, N.Y., 1984)), and is a homotrimer of the hexon protein (Roberts et al., Science, 232, 1148-1151 (1986)). The penton comprises a penton base, which is bound to other hexon capsomeres, and a fiber, which is noncovalently bound to, and projects from, the penton base. The penton fiber protein comprises three identical polypeptides (i.e., polypeptide IV). The Ad2 penton base protein comprises five identical polypeptides (i.e., polypeptide III) of 571 amino acids each (Boudin et al., Virology, 92, 125-138 (1979)).
The adenoviruses provide an elegant and efficient means of transferring therapeutic genes into cells. However, one problem encountered with the use of adenoviral vectors for gene transfer in vivo is the generation of antibodies to antigenic epitopes on adenoviral capsid proteins. If sufficient in titer, the antibodies can limit the ability of the vector to be used more than once as an effective gene transfer vehicle. For instance, animal studies demonstrate that intravenous or local administration (e.g., to the lung, heart or peritoneum) of an adenoviral type 2 or 5 gene transfer vector can result in the production of antibodies directed against the vector which prevent expression from the same serotype vector administered 1 to 2 weeks later (see, e.g., Yei et al., supra; Zabner (1994), supra; Setoguchi et al., Am. J. Respir. Cell. Mol. Biol., 10, 369-377 (1994); Kass-Eisler et al., Gene Therapy, 1, 395-402 (1994); Kass-Eisler et al., Gene Therapy 3, 154-162 (1996)). This is a drawback in adenoviral-mediated gene therapy, since many uses of an adenoviral vector (e.g., for prolonged gene therapy) require repeat administration inasmuch as the vector does not stably integrate into the host cell genome. The mechanism by which antibodies directed against an adenovirus are able to prevent or reduce expression of an adenoviral-encoded gene is unclear. However, the phenomenon is loosely referred to as "neutralization", and the responsible antibodies are termed "neutralizing antibodies."
There are three capsid structures against which neutralizing antibodies potentially can be elicited: fiber, penton, and hexon (Pettersson, supra). The hexon protein, and to a lesser extent the fiber protein, comprise the main antigenic determinants of the virus, and also determine the serotype specificity of the virus (Watson et al., J. Gen. Virol., 69, 525-535 (1988); Wolfort et al., J. Virol., 62, 2321-2328 (1988); Wolfort et al., J. Virol., 56, 896-903 (1985); Crawford-Miksza et al., supra). Researchers have examined and compared the structure of these coat proteins of different adenoviral serotypes in an effort to define the regions of the proteins against which neutralizing antibodies are elicited.
The Ad2 hexon trimer is comprised of a pseudohexagonal base and a triangular top formed of three towers (Roberts et al., supra; Athappilly et al., J. Mol. Biol., 242, 430-455 (1994)). The base pedestal consists of two tightly packed eight-stranded antiparallel beta barrels stabilized by an internal loop. The predominant regions in hexon protein against which neutralizing antibodies are directed appear to be in loops 1 and 2 (i.e., LI or l1, and LII or l2, respectively) in one of the three towers. For instance, Kinloch et al. (J. Biol. Chem., 258, 6431-6436 (1984)) compared adenoviral hexon sequences and theorized that the serotype-specific antigenic determinants on hexon are located in amino acid residues 120 to 470 encompassing the l1 and l2 loops since type-specific sequence differences are mainly concentrated in this region. Toogood et al. (J. Gen. Virol., 73, 1429-1435 (1992)) used peptides from this region to generate specific anti-loop antisera and confirmed that antibodies against residues 281-292 of l1 and against residues 441-455 of l2 were sufficient to neutralize infection. Also, Crompton et al. (J. Gen. Virol., 75, 133-139 (1994)) modified these loops to accept neutralizing epitopes from polio virus, and demonstrated that infection with the resultant adenoviral vector generated neutralizing immunity against polio virus. More recently it was demonstrated that the hexon protein is composed of seven discrete hypervariable regions in loops and 1 and 2 (HVR1 to HVR7) which vary in length and sequence between adenoviral serotypes (Crawford-Miksza et al., supra).
Less is known regarding the regions of the fiber protein against which neutralizing antibodies potentially can be directed. However, much data is available on the structure of the fiber protein. The trimeric fiber protein consists of a tail, a shaft, and a knob (Devaux et al., J. Molec. Biol., 215, 567-588 (1990)). The fiber shaft region is comprised of repeating 15 amino acid motifs, which are believed to form two alternating beta strands and beta bends (Green et al., EMBO J., 2, 1357-1365 (1983)). The overall length of the fiber shaft region and the number of 15 amino acid repeats differ between adenoviral serotypes. The receptor binding domain of the fiber protein and sequences necessary for fiber trimerization are localized in the knob region encoded by roughly the last 200 amino acids of the protein (Henry et al., J. Virol., 68(8), 5239-5246 (1994)); Xia et al., Structure, 2(12), 1259-1270 (1994)). Furthermore, all adenovirus serotypes appear to possess a type of specific moiety located in the knob region (Toogood et al., supra.)
Given the existence of these potential epitopes in hexon protein and fiber protein, it is understandable that, in some cases, difficulties have been encountered using adenovirus as a vector for gene therapy. Accordingly, recombinant adenoviral vectors capable of escaping such neutralizing antibodies (in the event they are preexisting and hamper gene expression commanded by adenovirus in an initial dose), and which would allow repeat doses of adenoviral vectors to be administered, would significantly advance current gene therapy methodology.
Thus, the present invention seeks to overcome at least some of the aforesaid problems of recombinant adenoviral gene therapy. In particular, it is an object of the present invention to provide a recombinant adenovirus comprising a chimeric coat protein that has a decreased ability or inability to be recognized by antibodies (i.e., neutralizing antibodies) directed against the corresponding wild-type adenovirus coat protein. These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.